Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning

Article abstract of DOI:10.1002/anie.201503148

Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and composition control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable composition and crystal facets. As model catalysts, the alloy NCs with different compositions showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the commercial Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.

Full text of DOI:10.1002/anie.201503148

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201503148
German Edition: DOI: 10.1002/ange.201503148
Hydrogenation Reactions
Pd–Pb Alloy Nanocrystals with Tailored Composition for
Semihydrogenation: Taking Advantage of Catalyst Poisoning**
Wenxin Niu, Yongjun Gao, Weiqing Zhang, Ning Yan,* and Xianmao Lu*
Abstract: Metallic nanocrystals (NCs) with well-defined sizes
and shapes represent a new family of model systems for
establishing structure–function relationships in heterogeneous
catalysis. Here in this study, we show that catalyst poisoning
can be utilized as an efficient strategy for nanocrystals shape
and composition control, as well as a way to tune the catalytic
activity of catalysts. Lead species, a well-known poison for
noble-metal catalysts, was investigated in the growth of Pd
NCs. We discovered that Pb atoms can be incorporated into the
lattice of Pd NCs and form Pd–Pb alloy NCs with tunable
composition and crystal facets. As model catalysts, the alloy
NCs with different compositions showed different selectivity in
the semihydrogenation of phenylacetylene. Pd–Pb alloy NCs
with better selectivity than that of the commercial Lindlar
catalyst were discovered. This study exemplified that the
poisoning effect in catalysis can be explored as efficient
shape-directing reagents in NC growth, and more importantly,
as a strategy to tailor the performance of catalysts with high
selectivity.
opportunity to design and synthesize highly active, selective,
and stable metallic NCs for catalysis.
In catalytic processes, uncontrolled poisoning by exposure
to a range of chemical compounds is one of the major
mechanisms for the deactivation of precious metal catalysts.[6]
Therefore, catalyst poisoning has raised considerable con-
cerns in academic and industrial research. For instance,
platinum-based catalytic converters for automobiles can be
poisoned if the vehicle is operated on gasoline containing lead
additives.[7] However, poisons are not always undesirable. A
catalyst can be intentionally poisoned to decrease its activity
toward an undesired reaction. A prominent example is the
use of lead compounds in the Lindlar catalyst which exhibits
ꢀ
=
high selectivity in the hydrogenation of C C to C C bonds by
blocking certain active sites.[8]
The strong interaction of poisoning species with noble-
metal catalysts enables a new family of shape-directing
reagents for the growth of metal NCs. For example, based
on its poisoning effect on Pt and Pd catalysts, carbon
monoxide was demonstrated to strongly impact the growth
of Pt and Pd NCs.[9] The Lindlar catalyst, a classic example of
catalyst poisoning, inspires us that Pb compounds may be
utilized as a shape-regulating reagent for the synthesis of Pd
NCs. Therefore, in this study, the function of PbII ions in the
growth of Pd NCs was systematically investigated. Unex-
pected formation of Pd–Pb alloy NCs with tunable shapes and
composition was first discovered and explained with a unique
alternative deposition mechanism. Moreover, the application
of these NCs in the hydrogenation of various alkynes was
demonstrated. Pd–Pb alloy NCs with higher selectivity for
semihydrogenation of alkynes than the commercial Lindlar
catalysts were demonstrated.
M
etallic nanocrystals (NCs) have found wide application as
heterogeneous catalysts in chemical and energy industries. In
heterogeneous catalysis, many significant reactions are highly
sensitive to the structure of the catalysts.[1] However, due to
the compositional and structural complexity of common
catalysts, a comprehensive understanding of the relationship
between the catalytic performance and their structures
remains a challenging goal.[2] Over the past few decades, the
rapid progress in the synthesis of metallic NCs has achieved
unprecedented success in controlling their sizes, shapes, and
compositions.[3] These research advances offer many degrees
of freedom for tuning their catalytic properties.[4] Moreover,
the resultant metallic NCs can be a new family of model
systems that provide the basis for elucidating structure–
function correlations in heterogeneous catalysis.[5] The syn-
ergy between fundamental structure–function research in
catalysis and new synthetic nanotechnologies will provide the
The effect of PbII ions on the growth of Pd NCs was
investigated through a seed-mediated growth process. Ascor-
bic acid (AA) was used to reduce H2PdCl4 on Pd cubic seeds
in the presence of cetyltrimethylammonium bromide
(CTAB). During the reduction process, different amounts of
Pb(NO3)2 solution were introduced to the growth solution. As
the concentration of Pb2+ increases, the shape of the NCs
gradually evolves from cube, to truncated cube, cuboctahe-
dron, truncated octahedron, and octahedron (Figure 1). All
these NCs are well-faceted and match well with correspond-
ing geometrical models. With PbII acetate as the PbII source,
similar shape transition was also observed (Figure S2),
suggesting Pb2+ is responsible for the shape transition of
these NCs.
[*] Dr. W. X. Niu, Dr. Y. J. Gao, Dr. W. Q. Zhang, Prof. N. Yan,
Prof. X. M. Lu
Department of Chemical and Biomolecular Engineering
National University of Singapore
4 Engineering Drive 4, Singapore 117585 (Singapore)
E-mail: ning.yan@nus.edu.sg
xianmao.lu@nus.edu.sg
[**] We thank the Ministry of Education Singapore (Grant no. R279-000-
391-112) and the Singapore National Research Foundation (Grant
no. R279-000-337-281) for financial support.
Transmission electron microscopy (TEM) studies of the
cubic, cuboctahedral, and octahedral NCs are shown in
Figure 2. Selected-area electron diffraction (SAED) and
high-resolution TEM (HRTEM) measurements indicate
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503148.
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Figure 1. Geometric models and scanning electron microscopy (SEM)
images of Pd-based NCs synthesized with different amounts of 10 mm
Pb(NO3)2: a,b) cubes, 0 mL; c,d) truncated cubes, 7 mL; e,f) cuboctahe-
dra, 7.5 mL; g,h) truncated octahedra, 10 mL; i,j) octahedra, 40 mL.
Scale bar: 100 nm.
Figure 3. a–c) XRD patterns of cubic, cuboctahedral, and octahedral
NCs. d) XPS spectra of Pb 4f region of Pd–Pb alloy NCs, respectively.
e) A plot of the surface atomic concentration of Pb (determined by
XPS) versus the molar ratio of Pb/Pd precursors. f) A plot of atomic
concentration (determined by EDX) of Pb versus the molar ratio of Pb/
Pd precursors.
abundance of {100} and {111} facets of these NCs, consistent
with TEM and SEM results. Interestingly, both (111) and
(200) peaks of the cuboctahedral NCs split into two peaks,
indicating the Pb atoms are not evenly distributed in the NCs.
The XRD pattern of the octahedral NCs show only one (111)
peak at a much smaller angle than that of the cuboctahedral
NCs.
XPS analysis shows that the surface concentration of Pb
on the alloy NCs can be increased to a plateau of 22–27% by
increasing the molar ratio of Pb/Pd precursors (Figure 3e).
This is close to a stoichiometric Pd3Pb phase, a stable phase in
the Pd–Pb phase diagram.[10] The binding energies of Pb 4f7/2
and 4f5/2 core levels are located at 136.8 and 141.6 eV
(Figure 3d), corresponding to metallic Pb.[11] EDX analysis
shows a similar trend in the overall atomic concentration of
Pb, with a plateau at about 15% (Figure 3 f). In the region of
low molar ratio of Pb/Pd precursors, the overall concentration
of Pb increases much faster than the surface concentration of
Pb, indicating that at this stage most of the Pb atoms are
located in the interior lattice of the NCs. EDX mapping
confirmed this assumption (Figure S3).
Figure 2. TEM images, SAED patterns, and HRTEM images of Pd-
based nanocrystals of three typical shapes : a–c) cubes synthesized in
the absence of Pb(NO3)2, d–f) cuboctahedra synthesized in the pres-
ence of 7.5 mL of 10 mm Pb(NO3)2, g–i) octahedra synthesized in the
presence of 40 mL of 10 mm Pb(NO3)2. Scale bars in (a,d,g) 100 nm,
(b,e,h) 50 nm, (c,f,i) 2 nm.
that all the NCs possess a continuous crystal lattice and are
highly crystalline. SAED and HRTEM images show that
cubic NCs are enclosed by {100} facets (Figure 2a–c). As the
volume of PbII solution increased to 7.5 mL, the corners of the
cubes became truncated and the shape of the NCs evolved to
cuboctahedra (Figure 2d–f), accompanied by an increased
percentage of {111} facet on the surface. When the volume of
PbII solution increased to 40 mL, the final NCs are octahedra
mainly enclosed by {111} facets (Figure 2g–i).
The composition of different NCs was determined by X-
ray diffraction (XRD), energy-dispersive X-ray (EDX), and
X-ray photoelectron spectroscopy (XPS) analyses. The XRD
peaks of both the cuboctahedral and octahedral NCs are
shifted to lower 2q values than those of pure Pd nanocubes
(Figure 3a–c), suggesting the formation of a Pd–Pb alloy. The
ratio between (111) and (200) peak density reflects the
Tailoring the composition of bimetallic alloy nanostruc-
tures provides great flexibility in engineering their proper-
ties.[12] However, the synthesis of alloy NCs with well-defined
shapes is especially challenging for metals with large potential
difference. Therefore, the formation of Pd/Pb alloy NCs with
well-defined shapes are quite unexpected because the poten-
2À
tial difference between Pb2+ and PdCl4 ions is over 0.7 V.
To elucidate the mechanism of the formation of Pd–Pb
alloy NCs, Pd nanocubes were added to a growth solution
containing CTAB, PbII ions, and AA. Size increase of the Pd
nanocubes was not observed after the reaction (Fig-
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ure S4a,b), suggesting AA cannot
cause the bulk reduction of Pb2+.
Nevertheless, XPS studies reveal the
existence of metallic Pb on the
surface of the nanocubes (Fig-
ure S4c), indicating that Pd nano-
cubes can catalyze the reduction of
Pb2+ and induce the formation of an
ultrathin layer of metallic Pb on the
their surfaces. However, such a thin
layer of Pb causes the poisoning of
the Pd nanocubes NCs and prevents
further reduction of Pb2+. The
strong interaction of Pd and ad-
sorbed Pb atoms[13] originates from
their strong metal–adatom bonding,
which is proportional to their large
electronegativity difference.[14]
When both PdCl42À and Pb2+ are
involved in the reaction, Pd atoms
can still be reduced on the surface of
Pb-poisoned NCs due to the high
reduction potential of PdCl4 ions.
After the deposition of Pd atoms,
the NCs are able to catalyze the
Figure 4. Hydrogenation of phenylacetylene on Pd–Pb NCs of different shapes and Pb surface
compositions: a) cubes, 0%; b) cuboctahedra, 0.2%; c) truncated octahedra, 2.1%; d) slightly
truncated octahedra, 13.8%; e) octahedra, 27.0%; f) octahedra, 0%.
2À
reduction of Pb2+ again. Such an alternating Pd- and Pb-
deposition process will repeat until the PbII or PdII species are
depleted. At low concentrations of Pb2+, the Pb atoms were
primarily incorporated in the interior crystal lattice of the
NCs, closer to the surface of the cubic seeds. Pd atoms
confined in the interior of the NCs are beyond the detection
depth of XPS techniques, therefore the surface concentration
of Pb in the cuboctahedral NCs is extremely low. When the
amount of Pb2+ increased to the range of 15–40 mL, Pb atoms
started to incorporate into the lattice close to the exterior
surface of the final NCs and thus the XPS signal increased
quickly until a stable Pb surface concentration of 22–27% was
reached. During the growth, the deposition of Pb atoms tends
to passivate the {111} facets, therefore as the concentration of
Pb2+ increases, the shapes of the NCs gradually change from
cubes with {100} facets to octahedra with {111} facets.
NCs with low surface Pb concentrations from 0–13.8%,
phenylacetylene was first hydrogenated to styrene, which was
further hydrogenated to ethylbenzene. In contrast, Pd–Pb
alloy octahedral NCs with 27.0% surface Pb dramatically
diminished the rate of subsequent hydrogenation of styrene
after phenylacetylene was depleted. The semihydrogenation
selectivity of Pd–Pb alloy octahedral NCs surpassed the
performance of Lindlar catalysts (91.4% vs. 81.1%, Fig-
ure S6). Moreover, the Pd–Pb alloy octahedral NCs show
even better selectivity at 508C. After a reaction for 24 h, the
Pd–Pb alloy octahedral NCs exhibited a selectivity of 99.1%,
whereas the selectivity of the Lindlar catalysts had dropped to
only 57.9%. The superior semihydrogenation performance of
the Pd–Pb alloy octahedral NCs can be generalized to
different alkynes (Table 1).
One of the most attracting properties of bimetallic alloy
NCs is their highly tunable catalytic properties.[15] The Pd–Pb
alloy NCs with tunable compositions are ideal catalysts for
establishing their structure–property relationship. Herein, the
composition-dependent properties of the Pd–Pb alloy NCs
were investigated in the semihydrogenation of phenylacety-
lene. The catalytic semihydrogenation of alkynes is of special
importance in the chemical industries.[16] Although Pd is one
of the most widely used catalysts for hydrogenation reac-
tions,[17] Pd usually exhibits low alkene selectivity in alkyne
hydrogenation.
Liquid-phase hydrogenation reaction of phenylacetylene
was first performed to examine the performance of different
Pd–Pb alloy NCs. Six samples of different shapes and surface
Pb concentrations were investigated: cubes 0%, cuboctahe-
dra 0.2%, truncated octahedra 2.1%, slightly truncated
octahedra 13.8%, octahedra 27.0%, octahedra 0%. The
selectivity of different catalysts was shown in Figure 4. For
Table 1: Selective semihydrogenation of alkynes using Pd–Pb alloy
octahedral NCs.[a]
Entry
Substrate
t [min]
Conv. [%] Yield [%]
Alkane
Alkene
1
2
3
4
5
2-butyne-1,4-diol
1-octyne
diphenylacetylene
1-decyne
210
210
210
210
300
96.8
99.0
96.0
100
95.9
0
13.1
0
0.6
2.5
96.8
85.9
96.0
99.4
93.4
1-phenyl-1-propyne
[a] Reaction conditions: substrate (0.4 mmol) and 0.5 mol% of Pd–Pb
alloy octahedral NCs in 2 mL ethanol were stirred at room temperature
under 1 atm hydrogen pressure.
The high selectivity of Pd–Pb alloy octahedral NCs in the
semihydrogenation process was investigated by Fourier trans-
form infrared spectroscopy (Figure S7). Compared with other
Pd and Pd–Pb alloy NCs with lower Pb concentration, the
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In summary, the poisoning effect of PbII ions in the
synthesis of Pd NCs was first demonstrated. A series of Pd–Pb
alloy NCs with tunable shapes and compositions were
synthesized, making use of the strong interaction of Pb
adatoms and the Pd surface. Moreover, the as-synthesized
alloy NCs were applied as model catalysts for the semi-
hydrogenation of alkynes. The Pd–Pb alloy octahedral NCs
showed remarkable selectivity for the semihydrogenation of
alkynes, which is better than that of the commercial Lindlar
catalyst. This study exemplified that the poisons in catalysis
can be explored as efficient shape-directing reagents in NC
growth and more importantly as a strategy to tailor the
performance of catalysts.
Keywords: catalyst poisoning · hydrogenation · Lindlar catalyst ·
metal nanocrystals · semihydrogenation
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Received: April 6, 2015
Published online: May 26, 2015
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